Thin, very high transmittance, back-illuminated, silicon-on-saphire semiconductor substrates bonded to fused silica

ABSTRACT

A very high transmittance, back-illuminated, silicon-on-thin sapphire-on-fused silica wafer substrate design is presented for enabling high quantum efficiency and high resolution, silicon or silicon-germanium avalanche photodiode detector arrays with improved indirect optical crosstalk suppression. The wafer substrate incorporates a stacked antireflective bilayer between the sapphire and silicon, comprised of single crystal aluminum nitride (AlN) and non-stoichiometric, silicon rich, amorphous silicon nitride (a-SiN X&lt;1.33 ), as well as a one quarter wavelength, magnesium fluoride (λ/4-MgF 2 ) back-side antireflective layer which is bonded to a fused silica wafer. The fused silica provides mechanical support, allowing the sapphire to be thinned to optimal thickness below 50 μm, for improved optical transmittance and in conjunction with monolithic sapphire microlenses, suppression of indirect optical crosstalk from multiple reflections of APD emitted light. After solid-state device fabrication, the silicon can be coated with photoresist and the fused silica dissolved in buffered hydrogen fluoride (HF) to recover the thin Si—(AlN/a-SiN X&lt;1.33 )-sapphire-(MgF 2 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX

Not Applicable

BACKGROUND OF THE INVENTION

For the purposes of imaging a scene, at low ambient illumination levels using a focal plane array comprised of solid-state detector devices, it is beneficial for the individual detectors in the array to be sensitive to light over a broad wavelength range and at single photon intensity levels. To support the fabrication of such an array of highly sensitive solid-state light detectors over broad spectral range, the back-illuminated semiconductor wafer substrate on which the detector devices are fabricated should be capable of transmitting light into the detectors with high efficiency from the ultraviolet to the midwave infrared for enabling high quantum efficiency detectors. The back-illumination approach of semiconductor detectors through the thin, transparent supporting wafer substrate is the preferred approach for fabricating large area, solid-state, imaging focal plane arrays due to a higher attainable quantum efficiency and sensitive-area-fill-factor for such detectors compared to front-illuminated detector arrays. Back-illuminated detector arrays can be flip-chip bump-bonded to CMOS readout integrated circuits (ROICs) that control detector operation. Using highly sensitive detector arrays of avalanche photodiodes (APDs) is the preferred approach for implementing compact, solid-state, single photon sensitive, focal plane array (FPA) imagers. The semiconductor wafer substrate should therefore support fabrication of light detecting elements with small pixel pitch for enabling high resolution arrays, by providing a means for direct and indirect optical crosstalk isolation between the APD detector elements. Optical crosstalk in solid-state avalanche detector arrays results from light emission during impact ionization that can erroneously trigger neighboring detector pixels in the array and increase the overall detector noise. Suppression of indirect optical crosstalk by multiple reflections in the substrate of APD emitted light, is improved by thinning the substrate as well as fabricating monolithic microlenses beneath the detector pixels. In addition, the semiconductor wafer substrate should support a high quality single crystal active semiconductor material layer with low defect density, in which light detecting elements characterized by low dark current can be fabricated.

The present invention describes a method for implementing a thin, very high transmittance, back-illuminated, silicon-on-sapphire semiconductor wafer substrate to support the fabrication of large scale, high quantum efficiency and high resolution silicon (Si) or silicon germanium (SiGe) avalanche photodiode focal plane array (APD-FPA) imagers, with improved indirect optical crosstalk suppression due to thinned sapphire and the possibility to fabricate monolithic microlenses in the sapphire. The method of the present invention allows thinning the back-illuminated sapphire below 50 μm thickness, which improves the optical transmittance into silicon through the sapphire and reduces indirect optical crosstalk from multiple reflections in the sapphire of APD emitted light, while supporting fabrication of monolithic sapphire microlenses beneath detectors that further helps to reduce indirect optical crosstalk. The monolithic sapphire microlenses also improve detector quantum efficiency and array sensitive-area-fill-factor. To support thinning the sapphire substrate below 50 μm thickness, the sapphire side of the silicon-on-sapphire substrate is bonded to a thicker, transparent fused silica plate that provides mechanical support while also allowing optical access to devices. Without, the bonded fused silica support substrate, thinning the sapphire below 50 μm thickness would result in a substrate too fragile and prone to breakage for normal processing in an automated silicon foundry. The diameter of the fused silica plate is not restricted to have the same diameter as the thin silicon-on-sapphire substrate, thereby providing flexibility to fabricate devices on the silicon-on-sapphire-on-fused silica wafers in foundries equipped with a wide range of tooling sizes not having to match the diameter of the silicon-on-sapphire wafers but only the bonded fused silica wafer diameter. After solid-state devices have been fabricated in the device silicon, the silicon can be coated with a protective photoresist layer, and the wafer immersed in a buffered hydrogen fluoride (HF) etching solution that dissolves the fused silica support wafer while leaving the thin, silicon-on-sapphire wafer substrate unaffected. The thin, very high transmittance silicon-on-sapphire-on-fused silica substrate incorporates an antireflective bilayer comprised of refractive index matched single crystal aluminum nitride (AlN) in conjunction with amorphous, non-stoichiometric, silicon rich, silicon nitride (a-SiN_(X<1.33)), designated as (AlN/a-SiN_(X<1.33)), between the sapphire and silicon to improve the transmittance of light from sapphire into the silicon detector device layer. An amorphous, one quarter wavelength magnesium fluoride (λ/4-MgF₂) antireflective layer deposited on the back surface of the thinned sapphire wafer is meant to provide refractive index matching and thereby improve the optical transmittance from the ambient into the sapphire. The magnesium fluoride coated surface of the high transmittance Si—(AlN/a-SiN_(X<1.33))-sapphire with λ/4-MgF₂ back-side antireflective layer can be bonded directly to fused silica, thereby allowing the sapphire to be thinned below 50 μm, to provide high, back-illuminated transmittance into the silicon and help suppress indirect optical crosstalk by multiple reflections of APD emitted light in the sapphire. Alternatively, the bare sapphire surface without a λ/4-MgF₂ layer can also be bonded directly to the fused silica support wafer. The single crystal AlN component of the antireflective bilayer can be grown by epitaxy in non-polar A-plane orientation on semipolar R-plane sapphire substrates or grown in polar C-plane orientation on lower cost C-plane sapphire substrates. The amorphous, non-stoichiometric a-SiN_(X<1.33) component of the antireflective bilayer can be deposited by low pressure chemical vapor deposition (LPCVD) for example, on a full thickness (100) silicon wafer substrate followed by wafer bonding of the non-stoichiometric a-SiN_(X<1.33) on (100) silicon, to the single crystal AlN-on-sapphire. The (100) silicon wafer can be subsequently thinned using lapping and polishing to an appropriate thickness for fabrication of detector devices.

Using non-stoichiometric a-SiN_(X<1.33) for the antireflective layer is necessary, to provide proper refractive index matching in conjunction with single crystal AlN, between the sapphire and silicon. The stoichiometric a-SiN_(X=1.33) or a-Si₃N₄ has a refractive index that is too low to provide adequate refractive index matching between the AlN and silicon. Moreover, non-stoichiometric, silicon-rich a-SiN_(X<1.33) has a lower tensile strain than stoichiometric a-SiN_(X=1.33) when deposited on silicon and this helps to reduce the injection of defects into the silicon device layer. In addition, although a-SiN_(X<1.33) is meant to function primarily as an antireflective layer in the present invention, it can also perform the important secondary function of passivating the (100) silicon surface and silicon bulk. If the a-SiN_(X<1.33) thin film layer is deposited at a sufficiently low temperature of 200-450° C. by plasma enhanced chemical vapor deposition (PECVD) or hot filament chemical vapor deposition (HFCVD) methods for example, and therefore still retains hydrogen from the gaseous precursor chemicals used for deposition, the hydrogen will diffuse out of the a-SiN_(X<1.33) layer and into the silicon to passivate the silicon surface and also the silicon semiconductor bulk defects.

The thin, (less than 50 μm thick), very high transmittance, back-illuminated silicon-(AlN/a-SiN_(X<1.33))-sapphire substrate with λ/4-MgF₂ back-side antireflective layer bonded to fused silica, readily supports fabrication of silicon mesa detectors with exposed (111) side planes and fixed crystallographic angle φ_(C)=54.7° between (100) and (111) silicon planes, by liquid anisotropic etching using tetramethyl ammonium hydroxide (TMAH) to define the mesa APD detector array. The AlN/a-SiN_(X<1.33) antireflective bilayer and sapphire act as natural etch stopping layers to enable fabrication of highly uniform silicon mesa pixels by crystallographic etching. In addition, monolithic sapphire microlenses can be fabricated beneath each mesa APD detector pixel in the thinned sapphire to focus light under the full height of the detector mesas. The space between mesa APD detector pixels can be filled by an aluminum (Al) or copper (Cu) metal anode grid, forming a common anode electrical connection at the base of each silicon detector mesa. The Al metal common anode grid also performs the important secondary function of providing direct optical crosstalk isolation between adjacent APD detectors by blocking light emitted during the avalanche gain process. The thin (less than 50 μm thick) sapphire substrate combined with monolithic microlenses helps to suppress indirect optical crosstalk due to multiple reflections of APD emitted light, thereby improving the sensitivity of the APD detectors in the high resolution array. After the devices have been fabricated, the thin, very high transmittance, back-illuminated Si—(AlN/a-SiN_(X<1.33))-sapphire with λ/4-MgF₂ back-side antireflective layer can be recovered by coating the silicon with a protective photoresist layer and dissolving the bonded fused silica support plate in buffered hydrofluoric (HF) acid solution.

In the relatively recent past, it has become possible to fabricate solid-state arrays of silicon avalanche photodiodes optimized to operate either in linear mode or in non-linear Geiger-mode and capable of providing single photon sensitivity over a wavelength range from ultraviolet (UV) to near infrared (NIR). Focal plane arrays comprised of such linear or Geiger-mode silicon APD detectors, however, have only been fabricated in less than optimal substrate material systems due to the unavailability of a fully optimized wafer substrate, inherently capable of supporting high broadband quantum efficiency and high resolution, back-illuminated detector arrays with monolithic microlenses and direct and indirect optical crosstalk isolation between pixels in the array. To date, no effective substrate technology exists that inherently provides nearly 100% broadband, back-illuminated optical transmittance into the silicon semiconductor detector device layer, while supporting small pixel pitch, high resolution APD detector arrays with monolithic microlenses and direct and indirect optical crosstalk isolation between pixels. The thin, very high transmittance, back-illuminated Si—(AlN/a-SiN_(X<1.33))-sapphire with λ/4-MgF₂ back-side antireflective layer bonded to a dissolvable, fused silica mechanical support wafer offers a fully optimized and versatile substrate solution for fabricating high quantum efficiency and high resolution APD arrays on thin sapphire with monolithic microlenses contributing to indirect optical crosstalk suppression.

As illustrated in U.S. Pat. No. 7,271,376, the design of a silicon avalanche photodiode control circuit and detector device uses a silicon-on-insulator wafer where the starting substrate is described to be either the handle wafer of a p-silicon-on-insulator wafer or a p-Si substrate with a SiO₂ insulating layer. The substrate design however, is not optimized to support maximum thinning by bonding to a dissolvable fused silica support wafer, to help provide nearly 100% back-illuminated optical transmittance into the APD detector as well as to provide direct and indirect optical crosstalk isolation that will enable high resolution arrays.

As illustrated in U.S. Pat. No. 7,268,339, a method is described for forming semiconductor avalanche photodiodes in an n-doped semiconductor wafer using a plurality of doped, opposing trenches in the top and bottom surfaces of the substrate wafer. The substrate design however, is not optimized to support maximum thinning by bonding to a dissolvable fused silica support wafer, to help provide nearly 100% back-illuminated optical transmittance into the APD detector as well as to provide direct and indirect optical crosstalk isolation that will enable high resolution arrays.

As illustrated in U.S. Pat. No. 6,864,965, the imaging focal plane array supports dual-mode operation in both passive and active detection modes using LADAR pulses for the active mode. Switching between detection modes is accomplished by increasing the voltage bias across the detector so as to increase the gain and therefore sensitivity to the active laser pulse returns from objects in a scene. The mercury-cadmium-telluride (HgCdTe) detector pixels are shown to be fabricated in a semiconductor substrate having an antireflective coating applied to the surface. The substrate design however, is not optimized to support maximum thinning by bonding to a dissolvable fused silica support wafer, to help provide nearly 100% back-illuminated optical transmittance into the APD detector as well as to provide direct and indirect optical crosstalk isolation that will enable high resolution arrays.

In another embodiment of a position sensitive solid-state detector with internal gain, U.S. Pat. No. 6,781,133 B2, the invention describes a detection device and signal readout scheme, that uses an n-silicon substrate that is doped with p-materials using deep diffusion. The substrate design however, is not optimized to support maximum thinning by bonding to a dissolvable fused silica support wafer, to help provide nearly 100% back-illuminated optical transmittance into the APD detector as well as to provide direct and indirect optical crosstalk isolation that will enable high resolution arrays.

In another embodiment, U.S. Pat. No. 5,892,575, a method and apparatus for imaging a scene are described for resolving the 3-D spatial structure in the scene. The light source emits pulses of laser light toward the object being imaged and the detector system includes an optical system and an array of light detectors operating in non-linear Geiger-mode. The optical system collects a portion of the light scattered off of objects in the scene and directs the collected light toward the array of light detectors. The invention refers to a monolithic array of light detectors operating in the non-linear Geiger-mode and does not describe a substrate design optimized to support maximum thinning by bonding to a dissolvable fused silica support wafer, to help provide nearly 100% back-illuminated optical transmittance into the APD detector as well as to provide direct and indirect optical crosstalk isolation that will enable high resolution arrays.

In another embodiment, U.S. Pat. No. 5,757,057, a method for fabricating a large array of avalanche photodiodes using a plurality of pixel contacts that are isolated electrically by one or more isolation structures is revealed. The avalanche photodiode pixels, however, are fabricated in a semiconductor substrate comprised of two oppositely doped layers and is not optimized to support maximum thinning by bonding to a dissolvable fused silica support wafer, to help provide nearly 100% back-illuminated optical transmittance into the APD detector as well as to provide direct and indirect optical crosstalk isolation that will enable high resolution arrays.

In another embodiment, U.S. Pat. No. 5,438,217, a planar avalanche photodiode device array is realized using a planar block of n-type semiconductor having a plurality of p-type wells in the block surrounded by a foundation of n-type semiconductor material. The substrate design however, is not optimized to support maximum thinning by bonding to a dissolvable fused silica support wafer, to help provide nearly 100% back-illuminated optical transmittance into the APD detector as well as to provide direct and indirect optical crosstalk isolation that will enable high resolution arrays.

In another embodiment, U.S. Pat. No. 4,177,084, a method for producing a low defect layer of silicon on a sapphire substrate is provided. The silicon-on-sapphire (SOS) wafer is formed by epitaxial deposition of (100) silicon directly on the R-plane sapphire substrate followed by implanting of Si ions near the higher defect density silicon-sapphire interface. Subsequent annealing of the amorphised silicon near the sapphire surface using the low defect density silicon near the top of the epitaxial layer as a template for regrowth, produces an improved quality silicon device layer near the sapphire. The substrate fabrication approach, however, does not describe a method to support maximum substrate thinning by bonding to a dissolvable fused silica support wafer, to help provide nearly 100% back-illuminated optical transmittance into the APD detector as well as to provide direct and indirect optical crosstalk isolation that will enable high resolution arrays.

In another embodiment, U.S. Pat. No. 5,441,591, a method for fabricating a silicon-on-sapphire wafer is provided using direct wafer bonding of a silicon wafer to a sapphire wafer by application of pressure and elevated temperature. The full thickness silicon wafer is subsequently thinned by chemical mechanical polishing to an appropriate thickness for fabricating semiconductor devices. The substrate design however, is not optimized to support maximum thinning by bonding to a dissolvable fused silica support wafer, to help provide nearly 100% back-illuminated optical transmittance into the APD detector as well as to provide direct and indirect optical crosstalk isolation between detectors that will enable high resolution arrays.

Note that the above solid-state, semiconductor, avalanche detectors and silicon-on-sapphire substrate fabrication approaches do not envision, nor describe a method for realizing an optimal thin substrate design bonded to fused silica, for silicon APD-FPAs, that helps provide nearly 100% broadband, back-illuminated optical transmittance into the APD detectors and provides an effective means for direct and indirect optical crosstalk isolation between detectors to enable high resolution arrays.

BRIEF SUMMARY OF THE INVENTION

The challenges associated with realizing an optimized semiconductor wafer substrate capable of supporting the fabrication of large scale, high quantum efficiency and high resolution, back-illuminated silicon (Si) and silicon-germanium (SiGe) avalanche photodiode (APD) arrays can be overcome by using an advanced, thin silicon-on-sapphire-on-fused silica material system. The main problem of transmitting broadband light through the transparent sapphire substrate into the silicon detector device layer in conventional silicon-on-sapphire (SOS) wafer substrates, occurs due to a thicker than optimum sapphire needed for providing mechanical support and the optical reflection losses occurring at the sapphire-silicon and the air-sapphire material interfaces, resulting from refractive index mismatch between the layers.

The advanced silicon-on-sapphire substrate that is the subject of the present invention solves the problem of thinning the sapphire to an optimum thickness needed for light transmission into silicon and the suppression of indirect optical crosstalk, by bonding the sapphire or λ/4-MgF₂ antireflective layer coating the sapphire back-side, to a dissolvable fused silica substrate. The fused silica substrate provides mechanical support for the optimally thinned sapphire, during solid-state device fabrication. After device fabrication the thin silicon-on-sapphire can be recovered by coating the silicon with a photoresist layer and immersing the wafer into buffered hydrogen fluoride (HF) etching solution that dissolves the fused silica, leaving the silicon-on-sapphire unaffected. To further improve the back-illuminated optical transmittance into silicon, the substrate incorporates an antireflective bilayer comprised of single crystal aluminum nitride (AlN) and amorphous, non-stoichiometric silicon nitride (a-SiN_(X<1.33)) referred to as (AlN/a-SiN_(X<1.33)) between sapphire and silicon. By selecting the proper composition for the non-stoichiometric, silicon rich a-SiN_(X<1.33) layer, thereby also tuning its refractive index, it becomes possible to provide nearly optimal refractive index matching in conjunction with the single crystal AlN, between the sapphire substrate and the silicon detector device layer. Selecting the proper thickness for the AlN as well as the proper thickness and stoichiometry for a-SiN_(X<1.33) will result in nearly optimal broadband optical transmittance from sapphire into silicon. An amorphous, one quarter wavelength magnesium fluoride (λ/4-MgF₂) antireflective layer deposited on the back surface of the thinned sapphire wafer improves the optical transmittance from the ambient into the sapphire.

In one embodiment of the invention, the single crystal AlN component of the (AlN/a-SiN_(X<1.33)) antireflective bilayer can be grown by epitaxy in A-plane orientation on an R-plane sapphire wafer substrate. In a second embodiment, lower cost C-plane sapphire substrates can be used to grow polar C-plane AlN antireflective layers. The amorphous, non-stoichiometric a-SiN_(X<1.33) component of the antireflective bilayer pair can be deposited on a full thickness (100) silicon wafer substrate followed by wafer bonding of the a-SiN_(X<1.33) on (100) silicon, to the single crystal AlN-on-sapphire. Using initially, the full thickness (100) silicon for mechanical support, the sapphire is thinned by lapping and polishing to optimal thickness of 10-50 μm for light transmission and indirect optical crosstalk suppression due to multiple reflections of APD emitted light. A λ/4-MgF₂ back-side antireflective layer can be deposited on the sapphire and subsequently bonded to a fused silica mechanical support wafer. The (100) silicon wafer can be subsequently thinned by lapping and polishing to an appropriate design thickness ranging from 10-50 μm for detector device fabrication.

The thin, very high transmittance, back-illuminated silicon-(AlN/a-SiN_(X<1.33))-sapphire substrate with λ/4-MgF₂ back-side antireflective layer bonded to fused silica, readily supports fabrication of silicon mesa detectors with exposed (111) side planes and fixed crystallographic angle φ_(C)=54.7° between (100) and (111) silicon planes, by liquid anisotropic etching using tetramethyl ammonium hydroxide (TMAH) to define the APD detector array. The AlN/a-SiN_(X<1.33) antireflective bilayer and sapphire act as natural etch stopping layers to enable fabrication of highly uniform silicon mesa pixels by crystallographic etching. In addition, monolithic sapphire microlenses can be fabricated beneath each mesa APD detector pixel in the thinned sapphire to focus light under the full height of the detector mesas and also contribute to indirect optical crosstalk suppression by multiple reflections in the sapphire of the APD emitted light. The space between mesa APD detector pixels can be filled by an aluminum (Al) or copper (Cu) metal anode grid, forming a common anode electrical connection at the base of each silicon detector mesa. The Al metal common anode grid also performs the important secondary function of providing optical crosstalk isolation for direct line of sight light propagation between adjacent APD detectors by blocking light emitted during the avalanche gain process.

The design of the thin, very high transmittance, back-illuminated silicon-(AlN/a-SiN_(X<1.33))-sapphire-(MgF₂)-fused silica substrate allows the complete substrate to be fabricated prior to the fabrication of Si or SiGe APD detectors in the silicon device layer, since none of the material layers in the substrate including AlN, a-SiN_(X<1.33), sapphire or MgF₂ will be adversely affected by subsequent APD detector device fabrication in the silicon mesas using conventional silicon processing technology (e.g. thermal oxidation, diffusion of dopant impurities, deposition and metallization steps). The fused silica might be etched slightly with the conventional silicon processing for devices that might include isotropic liquid etching with HF, however, this is not problematic since the fused silica will be dissolved in buffered HF after device fabrication, to recover the thin silicon-(AlN/a-SiN_(X<1.33))-sapphire-(MgF₂) substrate. The capability to fully fabricate and characterize the very high transmittance, back-illuminated silicon-(AlN/a-SiN_(X<1.33))-sapphire substrates with back-side λ/4-MgF₂ antireflective layer prior to fabricating the silicon detectors is cost effective, first, by allowing wafer substrates to be optimally fabricated, separately and independently from the detector devices, and second, by preventing the consumption of expensive semiconductor gases to manufacture detectors in unfinished wafer substrates that might not pass quality standards once fully fabricated.

In summary, an optimized thin, silicon-(AlN/a-SiN_(X<1.33))-sapphire substrate with back-side λ/4-MgF₂ antireflective layer bonded to dissolvable fused silica, having a properly designed thickness of AlN and MgF₂ layers as well as properly designed thickness and composition of a-SiN_(X<1.33), can achieve nearly 100% back-illuminated optical transmittance over a broad wavelength range from 250-1100 nm. The thin, very high transmittance substrate will support the fabrication of high quantum efficiency and high resolution, back-illuminated silicon and silicon-germanium APD detector arrays with effective direct and indirect optical crosstalk isolation between neighboring and distant pixels in the array. The thin, silicon-(AlN/a-SiN_(X<1.33))-sapphire substrate with back-side λ/4-MgF₂ antireflective layer bonded to fused silica can be fabricated separately and independently from the detector device arrays. Thus, there is no need to potentially waste expensive resources to manufacture detector arrays in unfinished wafer substrates that might not pass quality standards once fully fabricated.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

These and other features of the subject of the invention will be better understood with connection with the Detailed Description of the Invention in conjunction with the Drawings, of which:

FIG. 1 illustrates the stacking order and optimal thickness of material layers in the thin, very high transmittance, back-illuminated, silicon-(AlN/a-SiN_(X=0.62))-sapphire wafer substrate with λ/4-MgF₂ back-side antireflective layer bonded to a fused silica support wafer.

FIG. 2 illustrates the thin, very high transmittance, back-illuminated, silicon-(AlN/a-SiN_(X=0.62))-sapphire wafer substrate with λ/4-MgF₂ back-side antireflective layer bonded to a fused silica support plate and coated with protective photoresist layer for dissolving the fused silica support wafer in buffered hydrogen fluoride (HF) etching solution.

FIG. 3 illustrates recovery of the thin, very high transmittance, back-illuminated, silicon-(AlN/a-SiN_(X=0.62))-sapphire wafer substrate with λ/4-MgF₂ back-side antireflective layer after the bonded fused silica support wafer has been etched away and dissolved in buffered hydrogen fluoride (HF) etching solution leaving the thin Si—(AlN/a-SiN_(X=0.62))-sapphire-(MgF₂) unaffected.

FIG. 4 illustrates the stacking order and optimal thickness of material layers in the thin, very high transmittance, back-illuminated, silicon-(AlN/a-SiN_(X=0.62))-sapphire wafer directly bonded to a fused silica support plate with a direct sapphire to fused silica bond.

FIG. 5 illustrates a thin sapphire wafer directly bonded to a fused silica support plate with a direct sapphire to fused silica bond, thereby allowing sapphire to be thinned below 50 μm thickness using lapping and polishing or other means including chemical mechanical polishing for example.

FIG. 6 is a diagrammatic cross-sectional illustration of a square 27 μm side length, mesa APD detector pixel fabricated on thin, very high transmittance, back-illuminated, crystallographically etched, silicon-(AlN/a-SiN_(0.62))—(R-plane)-sapphire-(MgF₂) bonded to a dissolvable fused silica substrate, supporting direct and indirect optical crosstalk isolation between APD detectors and coated with a protective photoresist layer for dissolving the fused silica support wafer in buffered hydrogen fluoride (HF) etching solution.

FIG. 7 is a diagrammatic cross-sectional illustration of a square 27 μm side length, mesa APD detector pixel fabricated on thin, very high transmittance, back-illuminated, crystallographically etched, silicon-(AlN/a-SiN_(0.62))—(R-plane)-sapphire-(MgF₂) with microlenses, after the bonded fused silica support plate has been etched away and dissolved in buffered hydrogen fluoride (HF) etching solution leaving thin Si—(AlN/a-SiN_(X=0.62))-sapphire-(MgF₂) unaffected.

FIG. 8 illustrates the main components of a camera system utilizing a high sensitivity, solid-state, avalanche photodiode focal plane array imager, with mesa APD detectors fabricated on thin, very high transmittance, back-illuminated silicon-(AlN/a-SiN_(0.62))-sapphire substrate with λ/4-MgF₂ back-side antireflective layer, that is flip-chip bump-bonded to a CMOS readout integrated control circuit.

FIG. 9 is the calculated refractive index matching for the thin, very high transmittance, back-illuminated silicon-(AlN/a-SiN_(X))-thin sapphire wafer substrate with λ/4-MgF₂ back-side antireflective layer from FIG. 3, showing correct refractive index matching for a composition ratio of X=0.62 in silicon rich, silicon nitride.

FIG. 10 is the calculated absorption coefficient as a function of wavelength for the optimal composition, silicon rich a-SiN_(0.62) component of the antireflective bilayer between sapphire and silicon from FIG. 3, and confirms that minimal optical loss from absorption in the thin (<50 nm thick) a-SiN_(0.62) layer can be expected over the Tauc absorption region from 250 nm to λEg=590 nm.

FIG. 11 is the calculated, optical transmittance of the back-illuminated silicon-(AlN/a-SiN_(0.62))-sapphire wafer substrate with λ/4-MgF₂ back-side antireflective layer from FIG. 3, showing transmittance T>50% for 250-300 nm light, T>70% for 300-400 nm light and T>90% for 400-1100 nm light.

FIG. 12 illustrates the sequence of fabrication steps that can be used to implement or manufacture the thin, very high transmittance, back-illuminated, silicon-(AlN/a-SiN_(0.62))-sapphire wafer substrate with λ/4-MgF₂ back-side antireflective layer bonded to a dissolvable fused silica wafer from FIG. 1.

FIG. 13 illustrates the sequence of fabrication steps that can be used to implement or manufacture the thin, very high transmittance, back-illuminated, silicon-(AlN/a-SiN_(0.62))-sapphire wafer substrate with monolithic sapphire microlenses and λ/4-MgF₂ back-side antireflective layer bonded to a dissolvable fused silica wafer from FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a depiction of the thin, very high transmittance, back-illuminated, silicon-on-sapphire-on-fused silica semiconductor wafer substrate is shown in a non-scale rendering, consisting of an ordered stacking of material layers including the top most single crystal silicon (Si) semiconductor layer 10, non-stoichiometric, silicon rich, amorphous silicon nitride antireflective layer with nitrogen to silicon ratio N:Si=0.62 or a-SiN_(0.62) 11, single crystal aluminum nitride (AlN) antireflective layer 12, optically transparent single crystal thin sapphire (α-Al₂O₃) wafer substrate 13, back-side one quarter wavelength thick magnesium fluoride (λ/4-MgF₂) antireflective layer 14 bonded to a fused silica wafer of equal or larger diameter than the thin sapphire wafer substrate 13, providing mechanical support that allows thinning the sapphire substrate 13, below 50 μm thickness.

Referring to FIG. 2, the principal advantage of the back-illuminated, silicon 10-(AlN/a-SiN_(0.62)) 12/11-sapphire 13 substrates with λ/4-MgF₂ 14 back-side antireflective layer bonded to fused silica 15, is depicted. The solid-state detector devices are meant to be fabricated in the silicon 10 semiconductor layer, having a thickness of 10-50 μm or less. The fused silica support wafer 15, shown bonded to the MgF₂ antireflective layer material that coats the back-side of the thin sapphire 13, allows the wafer to be handled and processed normally in a silicon foundry, whereas without the fused silica 15, fabricating the thin less than 50 μm sapphire 13 layer would result in a wafer substrate too fragile and prone to breakage to be processed in a normal silicon foundry. After solid-state detector devices have been fabricated in the silicon 10, the silicon can be coated with a protective photoresist layer 16, and the wafer substrate etched with buffered hydrogen fluoride (HF) acid solution to etch and dissolve the fused silica (SiO₂) 15 mechanical support wafer, and recover the thin silicon-(AlN/a-SiN_(0.62))-sapphire-(MgF₂) wafer substrate since the MgF₂ 14, sapphire 13, AlN 12, a-SiN_(0.62) 11 and silicon 10 are either not affected by the buffered HF or etched very slowly compared to the fused silica 15.

Referring to FIG. 3, a photoresist layer 16, has allowed the thin, very high transmittance, back-illuminated silicon 10-(AlN/a-SiN_(0.62)) 12/11-sapphire 13 substrates with λ/4-MgF₂ 14 back-side antireflective layer to be recovered by etching with buffered hydrogen fluoride (HF) any exposed SiO₂ that was bonded previously to the λ/4-MgF₂ 14, coated thin sapphire 13. The recovered back-illuminated wafer substrate, can be directly illuminated by light 17 incident from the back-side of the substrate that traverses the optimal thickness material layers λ/4-MgF₂ 14, sapphire 13, AlN 12 and a-SiN_(0.62) 11 with minimal reflection and absorption losses, to enter the silicon 10 active semiconductor layer and impinge on the light detectors.

Referring to FIG. 4, a depiction of the thin, very high transmittance, back-illuminated, silicon-on-sapphire-on-fused silica semiconductor wafer substrate is shown in a non-scale rendering, consisting of an ordered stacking of material layers including the top most single crystal silicon (Si) semiconductor layer 10, non-stoichiometric, silicon rich, amorphous silicon nitride antireflective layer with nitrogen to silicon ratio N:Si=0.62 or a-SiN_(0.62) 11, single crystal aluminum nitride (AlN) antireflective layer 12, optically transparent single crystal thin sapphire (α-Al₂O₃) wafer substrate 13, bonded directly to a fused silica wafer 15 of equal or larger diameter than the thin sapphire wafer substrate 13, providing mechanical support that allows thinning the sapphire substrate 13, below 50 μm thickness. After solid-state detector devices have been fabricated in the silicon 10, the silicon can be coated with a protective photoresist layer and the wafer substrate etched with buffered hydrogen fluoride (HF) acid solution to etch and dissolve the fused silica (SiO₂) 15 mechanical support wafer, and recover the thin silicon-(AlN/a-SiN_(0.62))-sapphire wafer substrate since the sapphire 13, AlN 12, a-SiN_(0.62) 11 and silicon 10 are either not affected by the buffered HF or are etched very slowly compared to the fused silica 15.

Referring to FIG. 5, the sapphire substrate 13, having R-plane, C-plane, A-plane, M-plane or another crystal orientation, is bonded directly to a fused silica wafer 15, providing mechanical support to the sapphire 13 and allowing the sapphire to be thinned to a thickness below 50 μm for improved light transmission through the sapphire 13. Solid-state light emitting devices such as laser diodes, light emitting diodes (LEDs) or other devices can be grown epitaxially on the thin sapphire 13, using the closely lattice matched GaN/AlGaN/InGaN material system. The diameter of the fused silica 15 mechanical support wafer can be of equal or larger diameter than the sapphire 13 wafer substrate, providing the flexibility to process the sapphire wafers in foundries with tooling that matches the diameter of the fused silica 15 mechanical support wafer. After the fabrication of light emitting devices or other types of solid-state devices on the thin sapphire 13 bonded to fused silica 15, the devices fabricated on the front surface of the sapphire can be coated with a protective photoresist and the thin sapphire 13 with fabricated devices on the front surface, can be recovered by etching the fused silica using buffered hydrogen fluoride (HF) that leaves the sapphire unaffected.

Referring to FIG. 6, the principal advantage of very high transmittance, back-illuminated silicon 10-(AlN/a-SiN_(0.62)) 12/11-sapphire 13 substrates with λ/4-MgF₂ 14 back-side antireflective layer bonded to a fused silica 15 mechanical support wafer is depicted. The silicon avalanche photodiode (APD) 18 or other solid-state device can be fabricated in the silicon, followed by coating the silicon side of the wafer substrate with a protective photoresist 16 layer, and etching away the fused silica 15 mechanical support wafer in buffered hydrogen fluoride (HF) to recover the very high transmittance, back-illuminated silicon-on-sapphire substrate with (AlN/a-SiN_(0.62)) 12/11 antireflective bilayer between sapphire and silicon and λ/4-MgF₂ 14 back-side antireflective layer. The silicon mesa avalanche photodiode (APD) detector pixels 18 can be readily fabricated using anisotropic liquid etching with tetramethyl ammonium hydroxide (TMAH) for example, to define large arrays of highly uniform square based pyramidal silicon mesas, since the (AlN/a-SiN_(0.62)) 12/11-sapphire 13 substrate acts as a natural etch stopping layer. The anisotropically etched silicon pixel mesa 18 exhibits the characteristic angle φ_(c)=54.7 degrees 19 between the (100) 20 and (111) 21 silicon planes. A spherical, monolithic sapphire microlens 22 aligned beneath the silicon mesa pixel and fabricated using inductively coupled plasma etching (ICP) for example or other means, focuses light under the full height of the detector to compensate the reduced height silicon sidewalls. Dimensions of the pixel include square pixel side length 23 of a=27 μm, with corresponding diagonal dimension 24, sapphire thickness f=50 μm or less 25, mesa height h=10 μm 26, and microlens radius R_(ML)=53.59 μm 27. The back-surface of the sapphire substrate 13, which is contoured by the monolithic microlenses 22, is bonded to the fused silica 15 mechanical support wafer, by depositing an SiO₂ layer 28, on top of the λ/4-MgF₂ antireflective 14, and planarizing the SiO₂ using chemical mechanical polishing or other means, followed by bonding of the planarized SiO₂ 28, to the fused silica 15 (SiO₂) mechanical support wafer.

The avalanche photodiode (APD) detector is fabricated in the silicon mesa 18. The silicon APD cathode terminal 29 is electrically independent for each detector in the array. Silicon APD detectors however, share a common front-side anode contact 30, made locally at the base of each device mesa. A low resistance aluminum (Al) anode or copper (Cu) metal anode grid 31 fills the space between pixels and also functions to inhibit direct optical crosstalk by line of sight light propagation of APD emitted light, by shielding adjacent detectors from stray photons generated during the avalanche gain process. The figure also shows the R-plane surface of sapphire 32, tungsten vias 33, SiO₂ electrical isolation layer 34, n+ and p+ doped APD cathode and anode polysilicon 35 respectively and the p-type diffused high electric field APD avalanche region 36.

Thus, the silicon-(AlN/a-SiN_(0.62))-sapphire substrate with λ/4-MgF₂ back-side antireflective layer, supports an effective means for creating electrical isolation and direct optical crosstalk isolation by line of sight light propagation of APD emitted light between APD detectors at micrometer scales in an array, using crystallographic etching, to support low noise, high resolution, back-illuminated imaging focal plane arrays. The possibility of fabricating a monolithic sapphire microlens 22 in the sapphire substrate aligned beneath each mesa pixel, improves the APD detector quantum efficiency by focusing optical k-vectors under the full height of silicon and away from the optical dead space between pixels, thereby also providing 100% array sensitive-area-fill-factor. The sapphire microlens 22, also performs an important secondary function of suppressing indirect optical crosstalk in conjunction with a f≦50 μm 25 thin sapphire substrate 13 where APD emitted light instead of being reflected in the sapphire and transmitted into other APDs in the array, is redirected back into the APD 18, by the microlens 22.

Referring to FIG. 7, the very high transmittance, back-illuminated silicon 10-(AlN/a-SiN_(0.62)) 12/11-sapphire 13 substrate with λ/4-MgF₂ 14 back-side antireflective layer is recovered by coating the silicon side of the wafer with protective photoresist 16, and etching the fused silica mechanical support wafer with buffered hydrogen fluoride (HF). The silicon mesa avalanche photodiode (APD) detector pixels 18, fabricated using anisotropic liquid etching with tetramethyl ammonium hydroxide (TMAH) for example, to define large arrays of highly uniform square based pyramidal silicon mesas, with the (AlN/a-SiN_(0.62)) 12/11-sapphire 13 substrate acting as a natural etch stopping layer, are unaffected by etching of the fused silica mechanical substrate. The anisotropically etched silicon pixel mesa 18 exhibits the characteristic angle φ_(c)=54.7 degrees 19 between the (100) 20 and (111) 21 silicon planes. A spherical, monolithic sapphire microlens 22 with coated with a λ/4-MgF₂ 14 antireflective layer, aligned beneath the silicon mesa pixel 18 and fabricated using inductively coupled plasma etching (ICP) for example or other means, is not etched by the buffered hydrogen fluoride (HF). Dimensions of the pixel including the square pixel side length 23 of a=27 μm, with corresponding diagonal dimension 24, f=50 μm or less 25, mesa height h=10 μm 26, and microlens radius R_(ML)=53.59 μm 27, are unaltered and unaffected by etching of the fused silica, due to the protective photoresist 16. After etching the fused silica mechanical support wafer, the silicon APD 18 can be back-illuminated with minimal reflection loss, through the very high transmittance λ/4-MgF₂ 14 coated light focusing sapphire microlens 22 and very efficient AlN/a-SiN_(0.62) 12/11 antireflective bilayer between the sapphire 13 and silicon APD 18.

Referring to FIG. 8, the principal advantage of using a very high transmittance, back-illuminated, Si—(AlN/a-SiN_(0.62))-sapphire wafer substrate with λ/4-MgF₂ back-side antireflective layer for fabricating a solid-state APD detector array is depicted, where a high quantum efficiency and high resolution, all solid-state APD focal plane can be realized by use of a simple camera lens 36 with focal length F=0.21 meters 37, an aperture stop of diameter D=3.75 cm 38 and a focal plane array (FPA) consisting of a very high transmittance, back-illuminated Si—(AlN/aSiN_(0.62))-sapphire-(MgF₂) APD detector array chip 39 that is flip-chip bump-bonded to a CMOS readout integrated circuit (ROIC) 40. The light sensitive, square area of the FPA sets the field stop dimension 41, that in turn sets the angular field of view of the camera 42. The CMOS-ROIC 40 includes pixel circuits 43 and a top level metal anode ring 44 around the periphery of the pixel circuit array. The CMOS-ROIC 40 is shown wire bonded 45 to a ceramic interposer that is mounted to a thermoelectric cooler 46. The camera lens 36 allows the area in a scene subtending the field of view to be spatially conjugated back to the detector array.

The approach presented here for implementing a highly compact, back-illuminated, high quantum efficiency and high resolution, solid-state, APD focal plane array imager by using a thin, very high transmittance Si—(AlN/a-SiN_(0.62))-sapphire substrate with λ/4-MgF₂ back-side antireflective layer bonded directly to a fused silica mechanical support wafer, is novel. The design approach presented here, represents a significant advancement in high sensitivity, wide dynamic range, passive or active 2-D and 3-D imaging technology using large area, high quantum efficiency and high resolution arrays of silicon or silicon-germanium avalanche photodiodes that can operate in both linear mode and in single photon sensitive Geiger-mode. The advanced substrate design allows the sapphire to be thinned below 50 μm due to a bonded fused silica wafer that provides mechanical support to the thin Si—(AlN/a-SiN_(0.62))-sapphire substrate with λ/4-MgF₂ back-side antireflective layer, allowing the wafer substrate to be processed reliably in a silicon foundry without an increased risk of breakage due to thinning of the sapphire. After solid-state device fabrication in silicon, the thin, very high transmittance Si—(AlN/a-SiN_(0.62))-sapphire substrate with λ/4-MgF₂ back-side antireflective layer can be recovered by coating the silicon side of the wafer with protective photoresist and etching the wafer in buffered hydrogen fluoride (HF) to dissolve the fused silica (SiO₂) wafer while leaving the other materials of the wafer substrate unetched.

Referring now to FIG. 9, the refractive index as a function of optical wavelength given in micrometers is shown for the different material layers that comprise the very high transmittance, back-illuminated Si 47-(AlN/a-SiN_(0.62)) 49/48-sapphire 50 substrate with λ/4-MgF₂ 51 back-side antireflective layer. The fused silica (SiO₂) refractive index as a function of optical wavelength is not shown because the fused silica, bonded to the λ/4-MgF₂ coated sapphire is only present to provide mechanical support to the wafer substrate during solid-state device fabrication in a silicon foundry. After the solid-state devices have been fabricated, the fused silica wafer is etched with buffered hydrogen fluoride (HF). The principal design challenge for maximizing the back-illuminated optical transmittance for the substrate consists first, of adjusting correctly the thickness of the λ/4-MgF₂ 51 back-side antireflective layer to provide proper refractive index matching between the ambient or air, and sapphire and second, adjusting the thickness of AlN 49 and the thickness as well as the composition of silicon rich, non-stoichiometric a-SiN_(X<1.33), thereby also adjusting its refractive index 48, for optimal refractive index matching to be provided by the (AlN/a-SiN_(X<1.33)) antireflective bilayer between sapphire 50 and silicon 47 material layers.

Calculating the appropriate composition of the α-SiN_(X<1.33) component of the antireflective bilayer between sapphire and silicon that provides appropriate refractive index matching over a broad optical wavelength range in conjunction with AlN, can be accomplished by using the experimental results of Gardeniers et. al. who studied the material properties of non-stoichiometric, silicon rich, a-SiN_(X<1.33) films they deposited using low pressure chemical vapor deposition (LPCVD) from reaction of dichlorosilane (DCS) and ammonia (NH₃) at an elevated temperature between 700-900 degrees Celsius. Although their goal was to optimize the α-SiN_(X<1.33) thin films for microelectromechanical systems (MEMS) applications requiring low tensile strain, their results also confirmed an important theoretical model described by Makino and Stein et. al. separately, predicting the α-SiN_(X≦1.33) thin film refractive index as a function of the nitrogen to silicon ratio (x=N:Si) in the film. Their model assumes that the refractive index of a-SiN_(X≦1.33) films is a “bond-density-weighted linear combination” of amorphous silicon or a-Si and a-SiN_(1.33) reference refractive indices and is given as:

$n = \frac{{\left\lbrack {\left( {4/x} \right) - 3} \right\rbrack n_{0}} + {6n_{1.33}}}{\left( {4/x} \right) + 3}$

where n₀ represents the refractive index of a-Si and n_(1.33) represents the refractive index of stoichiometric a-SiN_(1.33). This equation in combination with the known refractive index as a function of wavelength curve or Sellmeier relation for stoichiometric a-SiN_(1.33) that provides the value of n_(1.33), may then be used to calculate the optimal value of the nitrogen to silicon ratio x=N:Si in a-SiN_(X) that provides optimal refractive index matching 48, between sapphire 50 and silicon 47 in conjunction with AlN 49. A calculated nearly optimal refractive index as a function of optical wavelength for the a-SiN_(X) component of the antireflective bilayer is provided when the nitrogen to silicon ratio in the layer is given as N:Si=0.62 or x=0.62 48 as shown in FIG. 9. Slight variations on the x=0.62, calculated value of the nitrogen to silicon ratio in a-SiN_(X) are also possible, in order for example to provide closer refractive index matching over a more limited wavelength range.

Referring now to FIG. 10, the absorption coefficient with units of 1/cm as a function of the optical wavelength in micrometers is calculated over the Tauc absorption region (from 250−λEg=590 nm 53) for the optimal composition a-SiN_(0.62) 52 component of the antireflective bilayer that provides refractive index matching between sapphire and silicon. It is necessary to be able to predict the absorption coefficient as a function of the optical wavelength 52 for the a-SiN_(0.62) material component of the antireflective bilayer, over the Tauc absorption region to verify its transparency near the short wavelength or high absorption region of the material. Ideally the a-SiN_(0.62) will be fully transparent and no light will be absorbed in the layer from 250-1100 nm.

Data for the extinction coefficient as a function of optical wavelength was not collected in the a-SiN_(X) samples that Gardeniers et. al. deposited at high temperature using LPCVD, however, it is still possible to infer a worst case scenario for the absorbance of the a-SiN_(0.62) component of the bilayer (from FIG. 9 48) using the data collected by Verlaan et. al. who used hot filament chemical vapor deposition (HFCVD) to deposit a-SiN_(0.62) which has identical stoichiometry to the nearly optimal antireflective layer (from FIG. 9 48). Verlaan et. al. measured the extinction coefficient of the HFCVD deposited a-SiN_(0.62) sample over the visible wavelength range from 400-650 nm 54. Although HFCVD as used by Verlaan et. al. maintained the substrate at a lower temperature of 230° C. during deposition compared to high temperature LPCVD used by Gardeniers et. al., the resulting a-SiN_(X) from HFCVD has a density approaching that of material deposited by high temperature LPCVD while retaining more hydrogen. Despite these differences between LPCVD and HFCVD deposited thin films, the lower quality HFCVD deposited a-SiN_(0.62) sample measured by Verlaan et. al. may be used to infer the worst expected case for the absorbance as a function of wavelength of the LPCVD deposited a-SiN_(0.62) material component of the antireflective bilayer (from FIG. 9 48).

Since Verlaan et. al. only measured the extinction coefficient for HFCVD deposited a-SiN_(0.62) from 400-650 nm 54, it is possible from their data to calculate the optical bandgap E_(g-opt) of a-SiN_(0.62) using the Tauc equation for amorphous materials to yield E_(g-opt)≈2.1 eV, and corresponding λ_(Eg)=590 nm 53, followed by fitting of the absorption coefficient of a-SiN_(0.62) as a function of wavelength from 250 nm to λ_(Eg)=590 nm, using the Tauc equation 52. The resulting predicted absorption as a function of wavelength curve for HFCVD deposited a-SiN_(0.62) 52 yields an absorption coefficient at 250 nm of α≈1×10⁵ 1/cm. This worst case result confirms that for the a-SiN_(0.62) component of the antireflective bilayer (from FIG. 9 48) if the thickness of the layer is maintained below 50 nm, the absorption losses in the layer can be disregarded.

Referring now to FIG. 11, the back-illuminated, optical transmittance 55 of the thin silicon-(AlN/a-SiN_(0.62))-sapphire substrate with λ/4-MgF₂ back-side antireflective layer from FIG. 3, is calculated using Monte Carlo integration and the wave transfer/scattering matrix method. It is assumed that the fused silica mechanical support wafer has been etched away with hydrogen fluoride (HF). Light is therefore incident directly upon the λ/4-MgF₂ coated thin sapphire and transmitted into the device silicon with minimal reflection loss through the AlN/a-SiN_(0.62) antireflective bilayer between sapphire and silicon. For optimized material layer thicknesses in the antireflective bilayer of 52 nm for AlN and 30 nm for a-SiN_(0.62), a sapphire substrate thickness of 50 μm or less and 120 nm thick λ/4-MgF₂ back-side antireflective layer, the result clearly shows high back-illuminated optical transmittance 55 from 250-1100 nm. The back-illuminated optical transmittance in the ultraviolet 56 exhibits T>50% for 250-300 nm, T>70% for 300-400 nm and T>90% from 400-1100 nm in the visible 57 and near infrared 58 regions of the electromagnetic spectrum.

With further adjustment of the material layer thicknesses including AlN, a-SiN_(0.62) and MgF₂ shown in FIG. 3, it is possible to optimize the back-illuminated optical transmittance 55 of the Si—(AlN/a-SiN_(0.62))-sapphire with λ/4-MgF₂ back-side antireflective layer to provide improved optical transmittance over specific wavelength ranges such as ultraviolet 56, visible 57, and near infrared 58. For example, an enhanced, nearly 100% back-illuminated optical transmittance can be achieved in the ultraviolet 56, at the cost of a reduced back-illuminated transmittance in the visible 57 and near infrared 58 wavelengths.

Referring to FIG. 12, the fabrication process for high transmittance, back-illuminated, silicon-(AlN/a-SiN_(0.62))-sapphire substrate with λ/4-MgF₂ back-side antireflective layer bonded to fused silica, is depicted up to and including the thinning of the silicon semiconductor active device layer by lapping and polishing, and before the anisotropic etching step of the silicon mesa array and the subsequent fabrication of the APDs. In one embodiment of the fabrication process for high transmittance, back-illuminated, silicon-(AlN/a-SiN_(0.62))-sapphire substrate with λ/4-MgF₂ back-side antireflective layer bonded directly to fused silica, the starting sapphire wafer substrate 13 is thinned by lapping and polishing means to thickness f=50-100 μm 25. The sapphire wafer substrate 13 crystal orientation can be either R-plane or C-plane for example. The single crystal aluminum nitride (AlN) material layer component 12, of the (AlN/a-SiN_(0.62)) antireflective bilayer is grown epitaxially to an appropriate thickness of 52 nm on the front surface of the thinned sapphire wafer substrate 13. The resulting single crystal AlN orientation can be A-plane if grown on R-plane sapphire or C-plane AlN if grown on C-plane sapphire. A full thickness l=750 μm 59, (100) silicon semiconductor wafer substrate 10 is used as a substrate to deposit the 30 nm thick non-stoichiometric, silicon rich, amorphous a-SiN_(0.62) 11 material layer component of the (AlN/a-SiN_(0.62)) antireflective bilayer. The a-SiN_(0.62) 11 face of the (100) silicon wafer substrate 10, is wafer bonded 60 to the AlN 12 face of the sapphire wafer substrate 13, the latter having thickness of 50-100 μm, using an appropriate gas ambient such as oxygen and elevated temperature with applied hydrostatic pressure, to help effect good molecular adhesion between the a-SiN_(0.62) 11 layer and AlN 12. Using the full thickness silicon wafer substrate to provide mechanical support, the bonded sapphire 13 substrate can be further thinned by lapping and polishing 61 or by other means to thickness f=50 μm or below. A 120 nm thick λ/4-MgF₂ 14 antireflective layer, can be deposited on the back surface of the thinned sapphire substrate 13 to provide refractive index matching between the ambient or air and sapphire 13. The sapphire 13 back surface with deposited λ/4-MgF₂ 14 antireflective is bonded to a fused silica 15 mechanical support wafer resulting in a direct bond 62 between MgF₂ 14 and the fused silica (SiO₂) 15. The full thickness (100) silicon wafer 10 can be subsequently thinned by lapping and polishing 61 for example, to an appropriate thickness of l=10-50 μm or less 59 to support fabrication of solid-state detector devices. After fabrication of solid-state devices in silicon 10, the wafer front-side can be coated with protective photoresist and the wafer substrate etched in buffered hydrogen fluoride (HF) for example, to dissolve the fused silica 15 and recover the Si—(AlN/a-SiN_(0.62))-thin sapphire 13 substrate with λ/4-MgF₂ 14 back-side antireflective layer since neither Si, AlN, a-SiN_(0.62), sapphire or MgF₂ are etched by the buffered HF.

Referring to FIG. 13, the fabrication process for high transmittance, back-illuminated, silicon-(AlN/a-SiN_(0.62))-sapphire substrate with monolithic sapphire microlenses coated with a λ/4-MgF₂ back-side antireflective layer bonded to fused silica, is depicted up to and including thinning of the silicon semiconductor active device layer by lapping and polishing, and before the anisotropic etching step of the silicon mesa array and the subsequent fabrication of the APDs. In one embodiment of the fabrication process for high transmittance, back-illuminated, silicon-(AlN/a-SiN_(0.62))-sapphire substrate with monolithic sapphire microlenses coated with a λ/4-MgF₂ back-side antireflective layer bonded directly to fused silica, the starting sapphire wafer substrate 13 is thinned by lapping and polishing means to thickness f=50-100 μm 25. The sapphire wafer substrate 13 crystal orientation can be either R-plane or C-plane for example. The single crystal aluminum nitride (AlN) material layer component 12, of the (AlN/a-SiN_(0.62)) antireflective bilayer is grown epitaxially to an appropriate thickness of 52 nm on the front surface of the thinned sapphire wafer substrate 13. The resulting single crystal AlN orientation can be A-plane if grown on R-plane sapphire or C-plane AlN if grown on C-plane sapphire. A full thickness l=750 μm 59, (100) silicon semiconductor wafer substrate 10 is used as a substrate to deposit the 30 nm thick non-stoichiometric, silicon rich, amorphous a-SiN_(0.62) 11 material layer component of the (AlN/a-SiN_(0.62)) antireflective bilayer. The a-SiN_(0.62) 11 face of the (100) silicon wafer substrate 10, is wafer bonded 60 to the AlN 12 face of the sapphire wafer substrate 13, the latter having thickness of 50-100 μm, using an appropriate gas ambient such as oxygen and elevated temperature with applied hydrostatic pressure, to help effect good molecular adhesion between the a-SiN_(0.62) 11 layer and AlN 12. Using the full thickness silicon wafer substrate to provide mechanical support, the bonded sapphire 13 substrate can be further thinned by lapping and polishing 61 or by other means to thickness f=50 μm or below. Monolithic sapphire microlenses 22 are subsequently fabricated in the thinned sapphire 13, using inductively plasma etching for example or by other means. A 120 nm thick λ/4-MgF₂ 14 antireflective layer, can be deposited on the back surface of the thinned sapphire substrate 13 matching the contour of the microlenses 22 to provide refractive index matching between the ambient or air and sapphire 13. A silicon dioxide (SiO₂) layer 28, can subsequently be deposited on the contoured λ/4-MgF₂ 14 antireflective layer followed by planarization using chemical mechanical polishing or other means and bonding of the planarized SiO₂ 28, to the fused silica 15 mechanical support wafer resulting in a direct SiO₂ to fused silica (SiO₂) bond 63. The full thickness (100) silicon wafer 10 can be subsequently thinned by lapping and polishing 61 for example, to an appropriate thickness of l=10-50 μm or less 59 to support fabrication of solid-state detector devices. After fabrication of solid-state devices in silicon 10, the wafer front-side can be coated with protective photoresist and the wafer substrate etched in buffered hydrogen fluoride (HF) for example, to dissolve the fused silica 15 and recover the Si—(AlN/a-SiN_(0.62))-thin sapphire 13 substrate with monolithic microlenses 22 coated with a contoured λ/4-MgF₂ back-side antireflective layer.

In summary, a novel method for implementing a very high transmittance, back-illuminated silicon-(AlN/a-SiN_(0.62))-sapphire substrate with λ/4-MgF₂ back-side antireflective layer, for enabling large scale, high quantum efficiency and high resolution APD imaging focal plane arrays has been presented and described. The said very high transmittance, back-illuminated, silicon-on-sapphire substrate design enables large scale arrays of high sensitivity and wide dynamic range silicon and silicon germanium APD-FPAs operating in dual linear mode and Geiger-mode, and also other types of back-illuminated, solid-state light detectors including CCDs and CMOS-APS devices. The previous limitations in the design of solid-state, back-illuminated, silicon APD imaging focal plane arrays have been overcome with the novel silicon-on-sapphire semiconductor wafer substrate design presented and described here, that supports very high optical transmittance into silicon and supports the fabrication monolithic sapphire microlenses that focus light under the full height of silicon detector mesas for high quantum efficiency and high sensitive-area-fill-factor arrays, and also supports an effective means for optical crosstalk isolation between adjacent and distant detectors, thereby enabling high resolution arrays. 

1. Apparatus consisting of a thin, very high transmittance, back-illuminated, silicon-on-sapphire semiconductor wafer substrate for enabling fabrication of high quantum efficiency and high resolution arrays of solid-state avalanche photodiode light detectors of micrometer scale for imaging focal plane arrays; and having a stacked antireflective bilayer between the sapphire and silicon layers of the substrate, comprised of single crystal AlN and non-stoichiometric, silicon-rich, amorphous a-SiN_(X<1.33) materials; and also having a one quarter wavelength thick, single layer magnesium fluoride (λ/4-MgF₂) antireflective layer deposited on the back surface of the sapphire substrate which in turn is bonded at elevated temperature with applied hydrostatic pressure, to a fused silica substrate wafer of equal or larger diameter, forming a direct bond between the magnesium fluoride and fused silica.
 2. The apparatus of claim 1, and further a method for transmitting optical power efficiently into the silicon semiconductor device layer of the wafer substrate, through the transparent sapphire layer, from the ambient, using a one quarter wavelength thick, single layer magnesium fluoride (λ/4-MgF₂) antireflective layer deposited on the back surface of the sapphire substrate, in conjunction with a stacked antireflective bilayer, comprised of single crystal AlN and non-stoichiometric, silicon-rich, amorphous a-SiN_(X<1.33) materials, between the sapphire and silicon layers of the said wafer substrate, where the sapphire can be thinned to below 50 μm thickness, enabled by the fused silica wafer bonded to the MgF₂ coated sapphire, providing mechanical support.
 3. The apparatus of claim 1, and further a method of transmitting optical power efficiently into the silicon semiconductor device layer of the wafer substrate through the transparent sapphire layer, using single crystal AlN and non-stoichiometric, silicon rich, amorphous a-SiN_(X<1.33) stacked successively in a bilayer arrangement between the sapphire and silicon layers in conjunction with a one quarter wavelength thick, single layer, magnesium fluoride (λ/4-MgF₂) antireflective layer deposited on the back surface of the thin, sapphire substrate which can be less than 50 μm thick, where the fused silica mechanical support wafer is removed after device fabrication in silicon, by coating the silicon side of the substrate with photoresist and etching for example in buffered hydrofluoric (HF) acid to dissolve the fused silica and recover the thin, back-illuminated silicon-on-sapphire substrate with AlN, a-SiN_(X<1.33) and MgF₂ antireflective layers which are either not etched, or only very slowly etched by the buffered hydrofluoric acid.
 4. The apparatus of claim 1, and further a method for transmitting optical power efficiently into the silicon semiconductor device layer of the wafer substrate, through the transparent sapphire layer, from the ambient, using a one quarter wavelength thick, single layer magnesium fluoride (λ/4-MgF₂) antireflective layer deposited on the back surface of the sapphire substrate, in conjunction with a stacked antireflective bilayer, comprised of single crystal AlN and non-stoichiometric, silicon-rich, amorphous a-SiN_(X<1.33) materials, between the sapphire and silicon layers of the said wafer substrate where the optical power originates from the ambient and propagates directly into the single layer magnesium fluoride (λ/4-MgF₂) antireflective layer since the fused silica has been etched away, followed by propagation into the thin, transparent sapphire substrate which can be less than 50 μm thick, followed by propagation into the single crystal AlN, followed by propagation into the a-SiN_(X<1.33) and lastly propagating into the silicon semiconductor layer of the said wafer substrate where the light can impinge upon the silicon or silicon-germanium light sensing elements.
 5. The apparatus of claim 1, and further a method of fabricating the thin, back-illuminated, silicon-on-sapphire wafer substrate having the AlN/a-SiN_(X<1.33) antireflective bilayer between the sapphire and silicon layers of the wafer substrate and a sapphire thickness which can be below 50 μm, where the sapphire face is bonded at elevated temperature with applied hydrostatic pressure, directly to the fused silica wafer, providing mechanical support, thereby allowing the silicon to be subsequently thinned by lapping and polishing or other means to an appropriate design thickness of 10-50 μm or thinner for solid-state device fabrication.
 6. The apparatus of claim 1, and further a method of fabricating a very thin sapphire substrate of either R-plane, C-plane, A-plane, M-plane or other crystalline orientation, by bonding at elevated temperature with applied hydrostatic pressure, the sapphire directly to a fused silica wafer providing mechanical support, which allows thinning the sapphire substrate below 50 μm thickness by lapping and polishing or other means.
 7. The apparatus of claim 1, and further a method of suppressing indirect optical crosstalk between solid-state avalanche photodiode light detectors of micrometer scale in the focal plane array due to multiple reflections in the sapphire substrate of the APD emitted light, by thinning the sapphire to 10 μm thickness, enabled by the bonded fused silica mechanical support wafer, and fabricating monolithic sapphire microlenses beneath the APD detectors that function not only to focus light into the APD but also in conjunction with the 10 μm thick sapphire, to redirect APD emitted light or optical k-vectors back into the detector, thereby preventing them from being reflected in the sapphire and transmitted to other, adjacent and distant APD detectors in the array.
 8. The apparatus of claim 1, and further a method of bonding at elevated temperature with applied hydrostatic pressure, the thin, 10 μm thick sapphire with monolithic sapphire microlenses and coated with a single layer magnesium fluoride (λ/4-MgF₂) antireflective layer contoured to the shape of the microlenses, of the silicon-on-sapphire substrate with stacked antireflective bilayer comprised of single crystal AlN and non-stoichiometric, silicon-rich, amorphous a-SiN_(X<1.33) materials, providing optimal refractive index matching between sapphire and silicon material layers, by depositing silicon dioxide (SiO₂) on top of the contoured magnesium fluoride (λ/4-MgF₂) antireflective layer and planarizing it by polishing or other means, followed by bonding at elevated temperature with applied hydrostatic pressure, of the planarized SiO₂ of the said substrate to a fused silica (SiO₂) support wafer that provides mechanical support during solid-state device fabrication in the wafer silicon, and can be subsequently etched away by coating the wafer silicon with photoresist before etching the fused silica using buffered hydrogen fluoride (HF) acid to dissolve the fused silica and recover the thin, back-illuminated silicon-on-sapphire substrate with monolithic microlenses and with AlN, a-SiN_(X<1.33) and MgF₂ antireflective layers which are not etched by the buffered hydrofluoric acid. 